Over the years a variety of communication systems have evolved which, to one degree or another, provide for reliable world-wide communications. VLF and ULF radio communication systems have long proven their worth, yet they have some limitations. Their susceptibility to jamming and their inability to penetrate seawater effectively have limited other usefulness for high priority communications.
The technological advances of space exploration have made available orbiting space platforms and recent advances in laser technology have provided a number of new communication devices. A marriage between these technologies logically is foreseen to provide a more reliable high-priority communication system.
Modulatable blue/green energy penetrates cloud covers and a considerable depth in seawater. The highly directability of blue/green lasers and their location on orbiting platform reduces the possibility of outside electromagnetic interference so that it would appear an orbiting laser system would satisfy the need for reliable wisespread communications.
Unfortunately, the technological expectations have not come to be. The current family of lasers and their related equipments lack the efficient, long-life, high-peak power, and high-average power requirements for space-qualified laser sources. High-transmission, wide-angle narrow-band optical filtering arrangements suitable for the subsurface platform also are lacking. Although the existing lasers and filters have some degree of flexibility, they are not refined to the point for optimizing transmission through different types of water. In addition, the state-of-the-art fails to provide for sufficient spacecraft offboard sight pointing of the narrow laser beam which would be directed at a specific submerged receiver. Similarly, most of the characteristics of a laser that enhance performance in conventional optical communications systems are so degraded by the satellite-to-subsurface propagation channel, that utilization of the advantages are difficult, the transmission channel through the clouds and seawater so degrades the nature of the transmitted laser pulse spatially, angularly and temporally that photon detection in the presence of background noise is difficult even with projected components. The nature of the cloud/ocean channel is foreseen to require additional receiver and transmitter complexity that might even further degrade performance. The channel adversely influences the system's performance, by, first, degrading the received pulse and by, second, degrading component capabilities required to detect the pulse.
Typically, the characteristics of a laser which normally serve to greatly enhance communication performance might be a disadvantage in communicating with widely scattered submerged receivers. The narrow spectral emission of the laser, the narrow pulse widths and high peak powers for discriminating against solar background, the narrow beam widths (small spot size) to concentrate energy density, and the narrow angular source size (light appearing to come from only one direction) could be considered as disadvantageous to a worldwide communication system that seeks to avoid betraying the submerged receivers' locations. Furthermore, the monochromatic nature of laser light allows high water transmission and a narrow optical prefiltering of solar background only if a narrow band, wide-angle filter can be constructed and, only if it can be obtained at the correct wavelength matching both the laser wavelength and the optimum water transmission wavelengths.
Collimated laser beams are useful only if a means exists to off-boresight point the beams and if the area coverage requirements permit their use and if the clouds do not further spread the beam. Enhancement due to "blue sky" Rayleigh scattering contributions at large zenith angles are also lost with small spot sizes. In like manner the narrow pulses possible with lasers permit gating out of most daytime background radiation but only if clouds do not stretch the pulse to values approaching 100 microseconds. The angular spreading due to both clouds and the ocean make the optical energy appear to come from virtually all forward directions which again severely limits narrowband detection. From the foregoing it is apparent that the orbiting of a modulatable laser light source poses formidable obstacles to actual implementation.
Thus, there is a continuing need in the state-of-the-art for a reliable optical communications system having regional and worldwide coverage which is relatively nonjammable and reliably transmits information through cloud cover and seawater to a submerged receiver.